During 12 weeks, 18 normal dogs were fed a high-caloric diet intended to induce obesity (weight-gain phase). For the next 12weeks (weight-loss phase), all dogs were fed a diet calculated to provide maintenance needs. During this second phase, dogs were randomly assigned to three groups differing only in their exercise regimen: group 1 dogs were not exercised, group 2 dogs were exercised three times each week on a treadmill, group 3 dogs were similarly exercised and outfitted with a vest holding additional weights. Echocardiographic data were obtained at baseline and following both the weight-gain and weight-loss phases. The weight-gain phase of the study was associated with an increase in bodyweight (31.4 per cent), decrease in body density (3.9 per cent) and an increase in left ventricular (LV) myocardial cross-sectional area in all groups. Cardiac hypertrophy was variably reversible during the weight-loss phase, with complete recovery for group 1, partial recovery for group 2 and no recovery in group 3. Regardless of group, weight loss was associated with a significant increase in LV diameter, a reduction of heart rate and an increase in heart rate-corrected isovolumetric relaxation time.